In recent years, electronic parts such as plasma display panels (PDPs) and integrated circuit (IC) chips generate more heat along with their increasing performance. This has led to the necessity of taking measures to prevent function failure due to temperature rises in the electronic parts of electronic devices.
General measures to prevent function failure due to temperature rise in an electronic part involve attaching a heat radiator such as a metallic heat sink, a radiation plate or a radiation fin to a heat source such as an electronic part to facilitate heat dissipation. When a heat radiator is used, the heat radiator and the heat source are closely attached to each other via a sheet member having high heat conductivity (heat conductive sheet) under a certain pressure in order to efficiently transfer heat from the heat source to the heat radiator. As the heat conductive sheet, a sheet molded using a composite material sheet having excellent heat conductivity is used. Such heat conductive sheets sandwiched between a heat source and a heat radiator during use are required to have high flexibility, as well as high heat conductivity. In order to improve the heat conductivity of the heat conductive sheet, it is necessary to lower the thermal resistance of the heat conductive sheet. The thermal resistance of the heat conductive sheet, when sandwiched between the heat generator and the heat sink, is considered to be the sum of the bulk thermal resistance, which is the thermal resistance of the heat conductive sheet itself, and the interfacial thermal resistance at the interface between the heat source and the heat radiator and the heat conductive sheet.
It is known that the thermal resistance of the material itself, that is, the bulk thermal resistance is expressed in relation to the thickness and heat conductivity of the material as:bulk thermal resistance(m2·K/W)=thickness of material(m)/heat conductivity of material(W/m·K).From this relational expression, in order to reduce the thermal resistance of the heat conductive sheet itself, that is, the bulk thermal resistance of the heat conductive sheet, it is necessary to reduce the thickness of the heat conductive sheet and to improve the heat conductivity of the heat conductive sheet. On the other hand, the interfacial thermal resistance of the heat conductive sheet is known to increase or decrease depending on the adhesion (interfacial adhesion) at the interface with the heat source and the heat radiator, the difference in bulk thermal resistance between the heat source and the heat conductive sheet, and the difference in bulk thermal resistance between the heat radiator and the heat conductive sheet. In particular, the interfacial adhesion can be affected by the pressure applied to the heat conductive sheet, the hardness (flexibility) of the heat conductive sheet, and the like. Therefore, in order to reduce the interface thermal resistance of the heat conductive sheet, it has been generally considered to increase the interfacial adhesion by giving a tack on the surface of the heat conductive sheet or decreasing the hardness of the heat conductive sheet.
For example, in WO2009/142290 (PTL 1), a primary sheet is formed from a composition containing a thermoplastic rubber, a thermosetting rubber, a thermosetting rubber curing agent, and an anisotropic graphite powder, and such primary sheets are stacked on top of each other to form a laminate, which in turn is sliced in the vertical direction to obtain a heat conductive sheet having a low thermal resistance of 0.04° C./W under a pressure of 0.5 MPa with graphite aligned in the vertical direction. In the heat conducting sheet, when forming a primary sheet from the composition, heat treatment is performed such that a thermosetting rubber is crosslinked with a thermosetting rubber curing agent and the thermosetting rubber crosslinked with the thermoplastic rubber coexist, thereby improving strength and flexibility.
For example, JP2014033193A (PTL 2) describes a heat conductive sheet containing a resin and a fibrous carbon fiber. By preventing 45 to 95% of the entire fibrous carbon fibers contained from being oriented in the thickness direction of the heat conductive sheet, the heat conductive sheet achieved a thermal resistance of 0.35° C./W under a pressure of 0.1 MPa.
For example, JP2006137930A (PTL 3) describes a heat conductive silicone grease composition containing a silicone oil and a copper powder having a low oxygen content. In the heat conductive silicone grease composition, the coarse particle mass is decreased by using a copper powder having a small average particle diameter and the coating thickness is made thin, whereby a sufficiently low thermal resistance of 0.036° C./W is achieved under a pressure of 0.15 MPa.